"The invention described herein was made in the performance of work under NASA Contract No. NAS 3-24631 and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of 1958 (72 Stat. 435; 42 U.S.C. 2457). However, the Government (NASA) has waived its patent rights in Waiver Case No. AW-2548."
1. Field of the Invention
The present invention generally relates to small propulsion systems for maneuvering spacecraft and, more particularly, is concerned with an arcjet thruster employing a starting control system and method for achieving a non-erosive arc initiation.
2. Description of the Prior Art
As conventionally known, an arcjet thruster converts electrical energy to thermal energy by heat transfer from an arc discharge to a flowing propellant and from thermal energy to directed kinetic energy by expansion of the heated propellant through a nozzle. For an explanation from a historical perspective of arcjet thruster construction and operation and the problems associated with this type of electrothermal propulsion, attention is directed to the following publications: "Arcjet Thruster for Space Propulsion" by L. E. Wallner and J. Czika, Jr., NASA Tech Note D-2868, June 1965; "The Arc Heated Thermal Jet Engine" by F. G. Penzig, AD 671501, Holloman Air Force Base, March 1966; and "Physics of Electric Propulsion" by R. G. Jahn, McGraw-Hill Book Company, 1968. Attention is also directed to U.S. Pat. No. 4,548,033 to G. L. Cann.
Most arcjet thrusters have as common features an anode in the form of a nozzle body and a cathode in the form of a cylindrical rod with a conical tip. The nozzle body has an arc chamber defined by a constrictor in a upstream portion of the body and a nozzle in a downstream portion thereof. The cathode rod is aligned on the longitudinal axis of the nozzle body with its conical tip extending into the upstream end of the arc chamber in spaced relation to the constrictor so as to defined a gap therebetween.
Although arcjet thrusters of this general configuration have been developed since the early 1960's, they have suffered extreme difficulty in starting and have associated therewith very high erosion rates on startup. Erosion of the nozzle body anode is due to initial movement of the foot of the arc across the gap and downstream along the constrictor of the nozzle body. Since flight applications of arcjet thrusters requires re-starting hundreds of times, erosions at starting can easily decrease the number of cycles that can be expected from the thruster.
Heretofore, starting the arcjet thruster has been accomplished by several different methods. In one method, a very high D.C. voltage is applied to the anode and cathode electrodes. In another approach, a high frequency A.C. voltage signal is used to initiate the arc. In still another method, a secondary electrode is used to initiate a small initial arc that then induces the main arc. However, none of these methods have been particularly successful since the flow of propellant gas is not controlled in such a way as to minimize anode erosion.
Consequently, a need exists for a fresh approach to arcjet thruster starting which will take into consideration all of the factors causing anode erosion at startup.